Contactless components or devices may be, for example, what are known as “NFC” components or devices, that is to say devices compatible with NFC (Near Field Communication) technology.
The NFC device may be, for example, an integrated circuit or a chip incorporating an NFC microcontroller.
The abbreviation NFC denotes a short-range, high-frequency wireless communication technology which allows data exchanges between two contactless devices over a short distance, for example 10 cm.
NFC technology is an open technology platform governed by the ISO/IEC 18092 and ISO/IEC 21481 standard, but incorporates many pre-existing standards such as the type A and type B protocols defined in the ISO-14443 standard, which may be communication protocols that can be used in NFC technology.
In addition to its conventional function as a telephone, a cellular mobile telephone may be used (if fitted with special circuitry) for exchanging data with another contactless device, for example a contactless reader, using a contactless communication protocol that can be used in NFC technology.
Other contactless devices, such as smartwatches, may also be mentioned.
These enable data to be exchanged between the contactless reader and secure elements located in the mobile telephone. Thus there are many possible applications, such as mobile ticketing in public transport (the mobile telephone acts as a transport ticket) or mobile payment (the mobile telephone acts as a payment card).
When data is transmitted between a reader and an object emulated in tag or card mode, the reader generates a magnetic field by means of its antenna, which, according to the conventionally used standards, is usually a sine wave at 13.56 MHz. The force of the magnetic field is between 0.5 and 7.5 amperes per meter RMS (“Root Mean Square”).
Two operating modes are then possible: a passive mode or an active mode.
In passive mode, only the reader generates the magnetic field, and the object, emulated in tag or card mode, is then passive and always acts as the target.
More precisely, the antenna of the object emulating the tag or the card modulates the field generated by the reader.
This modulation is performed by modifying the load connected to the terminals of the object's antenna.
When the load at the terminals of the object's antenna is modified, the output impedance of the reader's antenna changes because of the magnetic coupling between the two antennas. This causes a change in the amplitudes and/or phases of the voltages and currents present in the antennas of the reader and the object.
Thus the data to be transmitted from the object to the reader are transmitted by load modulation to the antenna currents of the reader.
The load variation carried out during the load modulation results in a modulation of the amplitude and/or phase of the signal (voltage or current) at the reader's antenna. A copy of the antenna current is generated and injected into the receiving circuitry of the reader, where this current is demodulated and processed to extract the transmitted data.
In the active operating mode, the reader and the object emulated in card mode both generate an electromagnetic field. This operating mode is generally used when the object is provided with its own power source, for example a battery, as is the case in a cellular mobile telephone which is then emulated in card mode.
Each of the NFC devices transmits the data using a modulation scheme.
Here again, the modulation results in a modification of the load, and we then speak of communication by active load modulation.
By comparison with a passive communication mode, this results in longer operating distances, possibly as long as 20 cm, depending on the protocol used.
Using active load modulation also enables very small antennas to be used.
However, this type of communication by active load modulation poses other problems.
During the active communication periods of the device emulated in card mode, the electromagnetic field of the reader is not directly observable. This may result in a non-synchronous response of the object emulated in card mode, and consequently the signal received by the reader may exhibit a phase shift, particularly in long periods of transmission by the device emulated in card mode.
This is even more apparent when the device emulated in card mode performs a digital modulation of the BPSK (Binary Phase-Shift Keying) type, and transmits the data to the reader using the type B communication protocol at a speed of 848 Kbits/s.
Additionally, to ensure that a phase-locked loop is stable, the use of a fixed frequency is recommended. For this purpose, it is best to restore some pulses of the received signal during a series of pulses. The received signal may therefore have a frequency up to 32 times lower, thus limiting the passband of the phase-locked loop. This limitation leads to instability in the system.
Furthermore, when the device emulated in card mode executes Manchester coding for transmitting data to the reader and uses the type A communication protocol at a speed of 106 kbit/s, this leads to a phase shift.
If there are two independent devices, namely the reader and an object emulated in card mode, capable of contactless communication by active load modulation, there is a need to minimize, or even suppress, this phase shift.